1. Field of the Invention
The present invention relates to a nitride semiconductor substrate obtained by growing a nitride semiconductor film such as gallium nitride (GaN) film on a base substrate, and to a method for manufacturing the same.
2. Description of the Related Art
GaN is a nitride semiconductor having Wurzite structure, and has a direct transition bandgap of 3.4 eV corresponding to the blue wavelength range of visible rays at room temperature, forms a solid solution with InN and AlN to control a bandgap energy, and exhibits a characteristic of a direct transition semiconductor over the entire composition range of the solid solution. Thus, GaN is widely used as a material for a light emitting device, in particular a blue light emitting device.
Generally, a GaN film is formed on a base substrate made of sapphire (Al2O3), silicon carbide (SiC) or silicon (Si) by MOCVD (Metal Organic Chemical Vapor Deposition) or HVPE (Hydride Vapor Phase Epitaxy) method. However, the base substrate and the GaN film have different lattice constants and thermal expansion coefficients, which makes it difficult to grow epitaxially the GaN film on the base substrate. This difficulty occurs to a nitride semiconductor including AlN, InN, GaInN, AlGaN and GaAlInN in the same way as GaN.
To overcome the difficulty, for reduction of lattice strain, a method is suggested to form a buffer layer with similar lattice constant on a base substrate at a relatively low temperature and grow a GaN film on the buffer layer.
However, disadvantageously this method uses a high cost base substrate and an additional growth apparatus to form the buffer layer. And, the method grows epitaxially the GaN film, but a dislocation density in the GaN film is still high. Thus, there is limitation in application to a laser diode or a light-emitting diode.
In the case of a sapphire base substrate, it is easy to grow a GaN film on the sapphire base substrate in the level of a current technique, however, to use the GaN film as a substrate of another device, the sapphire base substrate should be separated from the GaN film-grown substrate. That is, after the GaN film is grown on the sapphire base substrate, laser beams are illuminated on the sapphire base substrate, and the sapphire base substrate is separated from the GaN film by thermal decomposition. However, it requires much time to separate the sapphire base substrate and a separation yield is low.
To solve the above-mentioned problem, attempts have been made to grow a GaN film on an inexpensive silicon base substrate that is low cost and separate the silicon base substrate from the GaN film, thereby obtaining a GaN substrate. However, it is not easy to grow the GaN film on the silicon base substrate so far, and the silicon base substrate is often etched during GaN film growth. Further, although the GaN film is grown on the silicon substrate, warpage or cracks may occur to the base substrate due to differences in thermal expansion coefficient and lattice constant.
Meanwhile, Korean Patent No. 519326 suggests a technique for forming a plurality of grooves on a lower surface of a sapphire base substrate at a regular interval in a predetermined crystal orientation and forming a GaN layer on an upper surface of the sapphire base substrate to reduce minimum stress required to remove the sapphire base substrate after growing bulk gallium nitride, thereby reducing microcracks which may occur to the bulk gallium nitride and improving a crystal characteristic of the bulk gallium nitride. However, this patent uses the sapphire substrate as a base substrate for growing GaN, and thus requires a considerable time to separate the sapphire base substrate and has low separation yield. And, this patent forms the grooves on the lower surface of the sapphire base substrate to reduce stresses applied when separating the base substrate, however the inventors of the present invention ascertain that the grooves formed at a regular interval are ineffective in preventing warpage or cracks of the substrate due to difference in thermal expansion coefficient when growing the GaN film.